Regarding the interior......
Our boat had sat, growing mildew, for three years when we bought it as a fixer-upper.
My takeaway is that, after trying and failing to 'clean' the whole cushion, laundering the cushion foam is a waste of time. When it's old and flat and full of black mold, just toss it.
We did (cautiously) launder the covers several times in mild soap with some bleach. They did not fray or come apart, but that was probably the limit for that sort of therapy.
With new hi-density foam, and some zipper repairs, they went back in in '95, and still look, sit, and sleep good. The covers are -$$- expensive to replace unless you can sew your own. Note that we remove them to our dry and warm home every fall -- opens up the interior of the boat better.
Part 2: you have to clean under ALL interior parts..... cleaner and bleach. Get that black mold and dirt off the hull. Once deep-cleaned, you have to do a lesser cleaning about every other year. There will always be some moisture / condensation, and that attracts dirt and dirt attracts mold and mildew. So, no matter what boat you own, Cleanliness is a Good Thing.